Proteonematalycus wagneri Kethley reveals where the opisthosoma begins in acariform mites

It is generally thought that the anterior border of the opisthosoma of acariform mites is delineated by the disjugal furrow, but there is no evidence to support this other than the superficial appearance of tagmosis in some oribatids. It is proposed herein that the disjugal furrow is an apomorphic feature that does not correspond with any segmental borders. Although the disjugal furrow is absent from Proteonematalycus wagneri Kethley, the visible body segments of this species indicate that this furrow, when present, intersects the metapodosoma. Therefore, the disjugal furrow does not delineate the anterior border of the opisthosoma. Instead, this border is between segments D and E (segments VI and VII for all arachnids). This hypothesis can be accommodated by a new model in which the proterosoma warps upwards relative to the main body axis. This model, which is applicable to all Acariformes, if not all arachnids, explains the following phenomena: 1) the location of the gnathosomal neuromeres within the idiosoma; 2) the relatively posterior position of the paired eyes; 3) the shape of the synganglion; 4) the uneven distribution of legs in most species of acariform mites with elongate bodies.


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Grandjean based a widely cited model on van der Hammen's hypothesis to explain the 50 body segmentation of acariform mites [5]. According to this model, the dorsal region of the 51 podosoma is dramatically reduced, causing the body regions on either side to be pulled into 52 areas that were occupied by the podosoma (Fig. 1C). Klompen et al. also adhered to van der 53 Hammen's hypothesis when they suggested an amendment to Grandjean's model [6]. They 54 infer that the whole of the prosoma has warped somewhat evenly (Fig. 1D), which explains 55 how the dorsal surface of the podosoma can be completely diminished. These interpretations 56 are illustrated with an animation, which is accessible via the following link: 57 https://zenodo.org/record/5512807#.YUOPiflKhaQ.

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Proteonematalycus wagneri Kethley, a rare species of mite that is only known from sandy 59 habitats, is exceptionally useful for investigating the arrangement of body segments in 60 Acariformes. Unlike other basal acariform mites, in which the furrows that delimit the 61 segments are restricted to the dorsum, some of the furrows of P. wagneri completely encircle 62 the hysterosoma, clearly revealing which segments are associated with the metapodosoma.

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This mite was examined with a scanning electron microscope (SEM) and a light microscope 64 in order to confirm that these furrows are intersegmental.

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The integument of P. wagneri is extremely soft and fragile, causing it to readily distort when

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Despite the relatively low image resolution, light microscopy demonstrates how P. 85 wagneri appears when it has not undergone any shriveling. This species reveals very distinct 86 intersegmental furrows (Fig. 2C). This is an important distinction from mites that exhibit a 87 disjugal suture [9], which can be explained as a non-segmental feature because it bears no real 88 resemblance to an intersegmental furrow. A suture intersects a plane whereas an 89 intersegmental furrow is a slight constriction between two segmental humps (Fig. 3A).

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With respect to segmental homology, P. wagneri shows that segments C and D bear legs III 98 and IV, respectively, and so these segments represent the metapodosoma, whereas segment E 99 represents the first segment of the opisthosoma. There is no trace of a disjugal suture or 100 furrow.

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The disjugal suture 103 Other than the superficial appearance of tagmosis in some oribatids [4,5], there is no evidence 104 that the disjugal suture forms a boundary between the metapodosoma and opisthosoma. This 105 suture must instead intersect the metapodosoma (Fig.3B). Perhaps the main reason that van 106 der Hammen's hypothesis has been so widely adopted is that it adds two additional segments 107 to the acariform body (segments C and D are treated as separate from the metapodosoma),

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The presence of visible body segments, involving intersegmental furrows, is very obvious 123 in some soft-bodied basal acariform taxa [10,11]. This is also the case in other basal arachnid 124 lineages, for example, Mesothelae within Araneae [20,21], and Opilioacarida within 125 Parasitiformes [6,22]. It is therefore appropriate to base the interpretation of acariform body 126 segmentation on P. wagneri, which is a basal acariform mite with a complete set of 127 intersegmental furrows along the hysterosoma.

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A disjugal furrow is clearly absent from P. wagneri. There appears to be no example of a 129 true disjugal furrow, as opposed to a disjugal suture, in any mite (Fig. 3A). Instead, the  (Fig. 2), which is probably because of a constraint attributable to a 145 proterosomal warp (Fig 3C). But legs III and IV are much more posterior because they belong 146 on segments C and D. These segments can readily elongate because they are not constrained 147 by that warp. It is noteworthy that, primitively, the euarthropod body underwent tagmosis into 148 a head (proterosoma) and trunk (hysterosoma) [26,27]. Warping of the proterosoma may in 149 some way be linked to this tagmosis (Fig. 3C)